If this makes sense, it is not normal, but it is not unusual. Some bitches particularly of the larger breeds don't come in until they are about two. Of course she could have had a 'blind' season where you did not notice any colour or swelling, or maybe a very short one where she came in and went out without any signs.
I would not worry, just think yourself lucky you have not had to deal with all the hormones and the mess! You can get a chemical called Galastop which is supposed to bring them in, but unless there is a very good reason for doing this then I would prefer to let them come in naturally.
A friend of mine's bitch had one season at 17 months, they mated her, and she never had another season. I have a bitch who has blind seasons, and if I didnt have a male in the house I would never know. Unfortunately, she obviously still has the scent, but nothing else. So as Lankeela says, she may have had one and you missed it, or she may have one soon, or never....